We evaluated how gas-phase O interacts with residual petroleum hydrocarbons in soil. Total petroleum hydrocarbons (TPH) were 18 ± 0.6 g/kg soil, and TPH carbon constituted ∼40% of the dichloromethane-extractable carbon (DeOC) in the soil. At the benchmark dose of 3.4 kg O/kg initial TPH, TPH carbon was reduced by nearly 6 gC/kg soil (40%), which was accompanied by an increase of about 4 gC/kg soil in dissolved organic carbon (DOC) and a 4-fold increase in 5-day biochemical oxygen demand (BOD). Disrupting gas channeling in the soil improved mass transport of O to TPH bound to soil and increased TPH removal. Ozonation resulted in two measurable alterations of the composition of the organic carbon. First, part of DeOC was converted to DOC (∼4.1 gC/kg soil), 75% of which was not extractable by dichloromethane. Second, the DeOC containing saturates, aromatics, resins, and asphaltenes (SARA), was partially oxidized, resulting in a decline in saturates and aromatics, but increases in resins and asphaltenes. Ozone attack on resins, asphaltenes, and soil organic matter led to the production of NO, SO, and PO. The results illuminate the mechanisms by which ozone gas interacted with the weathered petroleum residuals in soil to generate soluble and biodegradable products.

We systematically assessed the removal of total petroleum hydrocarbons (TPHs) and other organic carbon using sequential biodegradation and ozonation in two soils (BM3 and BM4). We optimized the conditions for each step: 5% moisture content for ozonation and 10% moisture and circumneutral pH for biodegradation. For the relatively biodegradable TPH in BM3, preozonation and postozonation strategies were equally effective. In contrast, the more recalcitrant TPH in BM4 was better treated using postozonation. Carbon analyses along the treatment timeline revealed that dissolved organic carbon (DOC) was the dominant substrate for microbial consumption when readily biodegradable TPH was no longer available. The carbon in TPH was more reactive with O 3 than was the rest of dichloromethane-extractable organic carbon (DeOC), and the fate of TPH dictated the changes of DeOC. However, the fate of total organic carbon was controlled mainly by the microbial mineralization of DOC. Ozonation did not directly enhance the biodegradation rate of the residual TPH after ozonation, but ozone converted TPH into DOC that was microbially mineralized. This is the first study that compares the efficiencies of pre-and postozonation of two distinctly different soils. This study provides insights regarding the fundamental mechanism through which biodegradation and ozonation integrate and reveals the dynamics between ozonation and biodegradation during treatment petroleum-contaminated soils. Multistage ozonation+biodegradation is a useful tool to achieve the regulatory TPH standard (10,000 mg/kg), and DOC should be measured alongside TPH to gauge the dynamics of the integrated process.

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